Systems and methods for driving a light source

ABSTRACT

A circuit for driving a light source includes a voltage converter, a switch and a controller. The voltage converter converts an AC input voltage signal to a first rectified AC voltage signal. The voltage converter further generates an average signal proportional to an average voltage level of the first rectified AC voltage signal. The switch is coupled to the light source in series. The controller coupled to the voltage converter and the switch compares the first rectified AC voltage signal with the average signal to generate a pulse signal. The controller further generates a dimming control signal based on the pulse signal to control the switch thereby controlling dimming of the light source.

BACKGROUND

Furthermore, as shown in FIG. 5, a duty cycle D of the LPWM signal isgiven by:

$\begin{matrix}{{D = {{1 - \frac{2 \times T\; 1}{T}} = {{1 - \frac{2 \times T\; 1}{\left( \frac{1}{2 \times f\; 1} \right)}} = {1 - {4 \times f\; 1 \times T\; 1}}}}},} & (5)\end{matrix}$where T is a period of the LPWM signal. Since the product of f1 and T1is a constant value, the duty cycle D of the LPWM signal is alsosubstantially constant regardless of the amplitude and/or frequencyvariations in the AC input voltage VIN. In one embodiment, the dutycycle of the LPMW signal from the LPWM generator 430 is determined bythe resistors 311, 313, 315 and 317. By changing the resistance of theseresistors, the duty cycle of the LPWM signal can be adjusted to adapt tovarious applications, in one embodiment.

The waveform 509 in FIG. 5 represents an example of an LPWM signalgenerated by the LPWM generator 430. The waveform 510 in FIG. 5represents an example of a dimming control signal DIM output from theAND gate 409. When the signal V_(SIN) is less than the signal V_(DC),the LPWM signal is logic low (OFF period). During an OFF period, thedimming control signal DIM output is logic low such that the controlswitch 411 is switched off. There is no current flowing through the LEDstring 340 during the OFF period. When the signal V_(SIN) is greaterthan the signal V_(DC), the LPWM signal is logic high (ON period).During an ON period, a feedback signal FB indicative of a currentthrough the LED string 340 is compared to the predetermined referencesignal V_(REF). The dimming control signal DIM is determined by a pulsesignal generated by the frequency generator 440 and a comparison resultof the feedback signal FB and the predetermined reference signalV_(REF). By way of example, the pulse signal from the frequencygenerator 440 can have a frequency between approximately 300 KHz and 2.5MHz. Controlled by the dimming control signal DIM, the control switch411 can be switched on and off alternately to regulate the LED currentflowing through the LED string 340.

However, in the circuit 100 as shown in FIG. 1, only when the AC inputvoltage V_(IN) is higher than the voltage across the electrolyticcapacitor 105, the input current I_(IN) conducts through the rectifier103. Consequently, the input current I_(IN) represents a pulsatingcurrent waveform, which results in a poor power factor, e.g., 0.6.Additionally, the life time or mean time between failures (MTBF) of theelectrolytic capacitor 105 is much shorter than other elements of LEDdriving systems. As such, the electrolytic capacitor 105 dominates thelife time of such LED driving systems and therefore impairs theadvantage of long operating life of LED light sources.

SUMMARY

In one embodiment, a circuit for driving a light source includes avoltage converter (without an electrolytic capacitor), a switch and acontroller. The voltage converter converts an AC input voltage signal toa first rectified AC voltage signal. The voltage converter furthergenerates an average signal proportional to an average voltage level ofthe first rectified AC voltage signal. The switch is coupled to thelight source in series. The controller coupled to the voltage converterand the switch compares the first rectified AC voltage signal with theaverage signal to generate a pulse signal. The controller furthergenerates a dimming control signal based on the pulse signal to controlthe switch thereby controlling dimming of the light source.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the claimed subject matterwill become apparent as the following detailed description proceeds, andupon reference to the drawings, wherein like numerals depict like parts,and in which:

FIG. 1 is a block diagram of a conventional circuit for driving a load.

FIG. 2 is a block diagram of a driving circuit according to oneembodiment of the present invention.

FIG. 3 is an example of a schematic diagram of a driving circuitaccording to one embodiment of the present invention.

FIG. 4 is an example of a schematic diagram of a controller in FIG. 3according to one embodiment of the present invention.

FIG. 5 is an example of a timing diagram of signals generated by adriving circuit according to one embodiment of the present invention.

FIG. 6 is a flowchart of a method for driving a light source accordingto one embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the presentinvention. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims.

Furthermore, in the following detailed description of the presentinvention, numerous specific details are set forth in order to provide athorough understanding of the present invention. However, it will berecognized by one of ordinary skill in the art that the presentinvention may be practiced without these specific details. In otherinstances, well known methods, procedures, components, and circuits havenot been described in detail as not to unnecessarily obscure aspects ofthe present invention.

FIG. 2 illustrates a block diagram of a driving circuit 200 according toone embodiment of the invention. The driving circuit 200 provides aregulated DC output voltage V_(OUT) to a load, e.g., a light source 207.In one embodiment, the driving circuit 200 includes a power source 201,a voltage converter 203, and a controller 205. In one embodiment, thepower source 201 generates an AC input voltage V_(IN) signal having asinusoidal waveform. The voltage converter 203 converts an AC inputvoltage signal V_(IN) from the power source 201 to a regulated DC outputvoltage V_(OUT) to drive the light source 207. Furthermore, the voltageconverter 203 converts the AC input voltage signal V_(IN) to a firstvoltage signal V_(SIN) and a second voltage signal (average voltagesignal) V_(DC). More specifically, the voltage converter 203 can rectifyan AC input voltage signal V_(IN) to provide a rectified AC voltagesignal V_(REC). In one embodiment, the voltage converter 203 cangenerate a voltage signal V_(SIN) that is proportional to the V_(REC).The voltage converter 203 can also provide a DC voltage signal V_(DC)proportional to an average voltage level of the rectified AC voltagesignal V_(REC). Therefore, both V_(DC) and V_(SIN) can vary inaccordance with the AC voltage signal V_(IN).

The controller 205 receives the signals V_(SIN) and V_(DC) from thevoltage converter 203, and further receives a feedback signal FB fromthe light source 207 to generate a dimming control signal DIM. Morespecifically, the controller 205 can compare the signal V_(SIN) with thesignal V_(DC) to generate a pulse signal having a duty cycle. Thecontroller 205 can further provide a dimming control signal DIMaccording to the pulse signal and a feedback signal indicating a currentthrough the light source. The dimming control signal DIM is sent to thevoltage converter 203. Thus, the voltage converter 203 regulates powerdelivered to the light source 207 according to the dimming controlsignal DIM. In other words, the driving circuit 200 provides a dimmingcontrol function to adjust the light intensity of the light source 207.

Advantageously, since the signals V_(DC) and V_(SIN) are derived fromthe rectified AC voltage signal V_(REC), an electrolytic capacitor isremoved. By removing the electrolytic capacitor, the driving circuit 200offers a better power factor and a longer life time compared to theconventional driving circuit 100. Optionally, a ceramic capacitor can becoupled in parallel with the light source 207 to further reduce thenoise/ripple of the current through the light source 207.

Furthermore, since both V_(DC) and V_(SIN) vary in accordance with theAC input voltage signal V_(IN), a duty cycle of the pulse signal can bemaintained substantially constant even if the AC input voltage signalV_(IN) varies (such as amplitude or frequency variations). For example,if an amplitude of the AC input voltage signal V_(IN) varies from 85Vrms to 265 Vrms, the duty cycle of the pulse signal can still bemaintained substantially constant. Thus, the dimming control signal DIMis not affected by such variation and therefore a relatively constantillumination of the light source 207 can be achieved, which is describedin detail in relation to FIG. 3-FIG. 5.

FIG. 3 illustrates an example of a schematic diagram of a drivingcircuit 300 according to one embodiment of the present invention. Thedriving circuit 300 provides a regulated DC output voltage V_(OUT) to aload 340. For illustrative purposes, the load 340 includes three LEDscoupled in series. However, the invention is not so limited. The load340 may include other numbers of light sources and various types oflight sources. The driving circuit 300 includes a power source 310, avoltage converter 320, a controller 330, and a switch 321. The voltageconverter 320 further includes a voltage rectifier 350, a buck converter360, a capacitor 319, a first voltage divider including resistors 311and 313, and a second voltage divider including resistors 315 and 317.

In one embodiment, the power source 310 generates an AC input voltagesignal V_(IN) having a sinusoidal waveform. The voltage converter 320converts the AC input voltage signal V_(IN) to a regulated DC outputvoltage V_(OUT). More specifically, the voltage rectifier 350 includingdiodes D1 through D4 rectifies the AC input voltage V_(IN) signal toprovide a rectified AC voltage signal V_(REC). The voltage rectifier 350shown in FIG. 3 is a full-wave rectifier which outputs two peaks percycle of the AC input voltage signal V_(IN). As such, the repetitionrate of the rectified AC voltage signal V_(REC) is twice of the AC inputvoltage signal V_(IN). The buck converter 360 including a diode 323 andan inductor 325 further converts the rectified AC voltage signal V_(REC)into a regulated DC output voltage V_(OUT) suitable for driving the LEDstring 340.

Furthermore, a first voltage divider including the resistors 311 and 313further divides the rectified AC voltage signal V_(REC) to a dividedvoltage signal V_(SIN). The signal V_(SIN) is proportional to the signalV_(REC) and follows the waveform of the signal V_(REC). A second voltagedivider including the resistors 315 and 317 can cooperate with thecapacitor 319 to provide a voltage signal V_(DC) proportional to anaverage voltage level of the signal V_(REC). Thus, it can be inferredthat the signal V_(DC) is also proportional to an average voltage levelof the signal V_(SIN).

The signals V_(SIN) and V_(DC) are provided to the controller 330 via aVSIN pin and a VDC pin respectively. An HV_GATE pin of the controller330 is coupled to the gate of the switch 321, e.g., constructed of an Ntype metal-oxide-semiconductor field effect transistor (MOSFET), forproviding a constant DC voltage. In one embodiment, the constant DCvoltage is 15V. The drain of the switch 321 is coupled to the LED string340. The source of the switch 321 is coupled to a DRAIN pin of thecontroller 330 and to a VDD pin of the controller 330 through a diode327. Driven by the constant DC voltage from the HV_GATE pin, the switch321 is switched on. The VDD pin obtains a startup voltage derived from asource voltage at the source of the switch 321. The startup voltageenables the operation of the controller 330.

In one embodiment, the controller 330 compares the signals V_(SIN) andV_(DC) to control a conduction state of a switch (shown as switch 411 inFIG. 4) in the controller 330, whose drain and source are coupled to theDRAIN pin and the SOURCE pin respectively. In another embodiment, theswitch can also be located outside the controller 330. In bothcircumstances, the switch controls power delivered from the voltageconverter 320 to the LED string 340, thereby controlling the dimming ofthe LED string 340. Furthermore, a duty cycle representing a ratio ofthe ON time duration of the switch to a total ON and OFF time durationof the switch in a cycle is not affected by the amplitude and/orfrequency variations in the AC input voltage V_(IN), in one embodiment.Thus, the LED string 340 can provide a relatively constant illuminationeven if the AC input voltage V_(IN) varies.

In one embodiment, the resistor 329 senses an LED current flowingthrough the LED string 340 to generate a feedback signal FB. The SOURCEpin of the controller 330 receives the feedback signal FB indicative ofthe LED current. The controller 330 controls the conduction state of theswitch (shown as switch 411 in FIG. 4) based on the feedback signal FBto regulate the LED current. Accordingly, the light intensity of the LEDstring 340 is adjusted based on the feedback signal FB. The RT pin ofthe controller 330 is coupled to a resistor 331 to determine anoperating frequency of the controller 330. The GND pin of the controller330 is coupled to ground.

As presented above, the driving circuit 300 is not equipped with anelectrolytic capacitor and therefore offers a better power factor and alonger life time. Furthermore, even if the AC input voltage V_(IN)varies its amplitude and/or its frequency, a relatively constantillumination of the LED string 340 can still be achieved.

FIG. 4 illustrates an example of a circuit diagram of the controller 330according to one embodiment of present invention. Elements labeled thesame in FIG. 3 have similar functions. FIG. 4 is described incombination with FIG. 3. The controller 330 includes a startup circuit410, a Zener diode 420, a pulse width modulation generator such as alateral pulse width modulation (LPWM) generator 430, a frequencygenerator 440, a comparator 403, a flip-flop 405 (e.g., a set-resetflip-flop), a driver 407, an AND gate 409, and a switch 411.

The Zener diode 420 generates a constant DC voltage at the HV_GATE pin.Driven by a constant DC voltage, the switch 321 is conducted and allowsa startup voltage to be generated at the VDD pin. The startup circuit410 receives the startup voltage and provides power to other electricelements to enable operation of the controller 330 if the startupvoltage at the VDD pin reaches a predetermined startup voltage level ofthe controller 330. The LPWM generator 430, including a comparator 401,compares the signal V_(SIN) at the VSIN pin with the signal V_(DC) atthe VDC pin. The LPWM generator 430 outputs a lateral pulse widthmodulation (LPWM) signal based on a comparison result of the signalsV_(SIN) and V_(DC). Since the signal V_(DC) is proportional to anaverage voltage level of the rectified AC voltage signal V_(REC), andthe signal V_(SIN) is proportional to the rectified AC voltage signalV_(REC), both signals V_(DC) and V_(SIN) vary in accordance with thesignal V_(REC). In other words, both signals V_(DC) and V_(SIN) vary inaccordance with the AC input voltage signal V_(IN). As a result, theLPWM signal can have a substantially constant duty cycle even if anamplitude and/or a frequency of the AC input voltage signal V_(IN)varies. As used herein, “substantially constant” means that the dutycycle of the LPWM signal may vary slightly due to a waveform distortionof the AC input voltage signal V_(IN) (e.g., caused by a utilitycompany) or due to non-ideality of the circuit components, but within arange such that the LED string 340 produces a relatively constantbrightness.

Furthermore, in the example of FIG. 4, the frequency generator 440generates a pulse signal which has a frequency determined by theresistor 331. Of course, the frequency generator 440 can be modifiedsuch that it generates a pulse signal which has a frequency determinedby a capacitor. Thus, in one embodiment, the pulse signal from thefrequency generator 440 can have a fixed frequency. An S-R flip-flop 405receives the pulse signal via an S pin. An R pin of the S-R flip-flop405 is coupled to an output of the comparator 403 which compares thefeedback signal FB at the SOURCE pin of the controller 330 with apredetermined reference signal V_(RFE), e.g., 0.25V. In one embodiment,the predetermined reference signal V_(RFE) indicates a peak currentflowing through the LED string 340. Therefore, in one embodiment, if thefeedback signal FB is greater than the predetermined reference signalV_(RFE), the S-R flip-flop is reset. A Q pin S-R flip-flop 405 iscoupled to an AND gate 409 through a driver 407 to provide a pulsesignal controlled by the output of the comparator 403. The AND gate 409also receives the LPWM signal from the LPWM generator 430.

A dimming control signal DIM output from the AND gate 409 controls theconduction state of the control switch 411. Referring to FIG. 3 and FIG.4, the control switch 411 is coupled in series with the LED string 340when the switch 411 is switched on. As such, the dimming control signalDIM controls the power delivered from the voltage converter 320 to theLED string 340 by turning the control switch 411 on and off alternately.Furthermore, the dimming control signal DIM is determined by both of thecomparison between the signals V_(SIN) and V_(DC) and the comparisonbetween the feedback signal FB and the predetermined reference signalV_(REF).

FIG. 5 illustrates an example of a timing diagram 500 of signalsgenerated by a driving circuit according to one embodiment of thepresent invention. FIG. 5 is described in combination with FIG. 4. Forillustrative purposes, the AC input voltage V_(IN) is a sinusoidalwaveform signal having a frequency of f₁ Hz (e.g., 60 Hz) and has arange from 85 Vrms to 265 Vrms. The waveform 501 represents the signalV_(SIN) derived from the 85 Vrms AC input voltage V_(IN) and thewaveform 505 represents the signal V_(SIN) derived from the 265 Vrms ACinput voltage V_(IN). The line 503 represents the signal V_(DC) derivedfrom the 85 Vrms AC input voltage V_(IN) and the line 507 represents thesignal V_(DC) derived from 265 Vrms AC input voltage V_(IN).

Assuming that the signal V_(SIN) reaches the signal V_(DC) at time T1,the time T1 can be given by:Sin(2πf ₁ T1)=V _(DC) /V _(SIN-PK),  (1)where V_(SIN-PK) represents the peak voltage of the signal V_(SIN).Referring back to FIG. 3, since the signal V_(SIN) is obtained bydividing the signal V_(REC) by the resistors 311 and 313, the peakvoltage V_(SIN-PK) of the signal V_(SIN) can be given by:

$\begin{matrix}{{V_{{SIN} - {PK}} = {\left( \frac{R\; 2}{{R\; 1} + {R\; 2}} \right) \times V_{{REC} - {PK}}}},} & (2)\end{matrix}$where R1 is the resistance of the resistor 311, R2 is the resistance ofthe resistor 313, and V_(REC-PK) is the peak voltage of the signalV_(REC). In the example of FIG. 3, the signal V_(DC) represents anaverage voltage level of a signal that is divided from the signalV_(REC) by the resistors 315 and 317. Thus, the signal V_(DC) can begiven by:

$\begin{matrix}{{V_{DC} = {\frac{2}{\pi} \times \left( \frac{R\; 4}{{R\; 3} + {R\; 4}} \right) \times V_{{REC} - {PK}}}},} & (3)\end{matrix}$where R3 is the resistance of the resistor 315 and R4 is the resistanceof the resistor 317. Therefore, combining equations (1), (2) and (3),the time T1 can be given by:

$\begin{matrix}{{{Sin}\left( {2\;\pi\; f_{1}T\; 1} \right)} = {\frac{2}{\pi} \times {\frac{\left( \frac{R\; 4}{{R\; 3} + {R\; 4}} \right)}{\left( \frac{R\; 2}{{R\; 1} + {R\; 2}} \right)}.}}} & (4)\end{matrix}$According to equation (4), a product of f1 and T1 maintainssubstantially constant regardless of the amplitude and/or frequencyvariations in the AC input voltage V_(IN).

Furthermore, as shown in FIG. 5, a duty cycle D of the LPWM signal isgiven by:

$\begin{matrix}{{D = {{1 - \frac{2 \times T\; 1}{T}} = {{1 - \frac{2 \times T\; 1}{\left( \frac{1}{2 \times f\; 1} \right)}} = {1 - {4 \times f\; 1 \times T\; 1}}}}},} & (5)\end{matrix}$where T is a period of the LPWM signal. Since the product of f1 and T1in is a constant value, the duty cycle D of the LPWM signal is alsosubstantially constant regardless of the amplitude and/or frequencyvariations in the AC input voltage V_(IN). In one embodiment, the dutycycle of the LPMW signal from the LPWM generator 430 is determined bythe resistors 311, 313, 315 and 317. By changing the resistance of theseresistors, the duty cycle of the LPWM signal can be adjusted to adapt tovarious applications, in one embodiment.

Assuming that the frequency of the AC input voltage V_(IN) is constant,the waveforms in FIG. 5 also illustrate that the duty cycle of the LPWMsignal remains constant even if the amplitude of the AC input voltageV_(IN) varies. The line 503 intersects with the waveform 501 at time T1,T2, T3, T4, T5 and T6, and the line 507 intersects with the waveform 505at the same time T1, T2, T3, T4, T5 and T6. Therefore, the intersectionsbetween the signal V_(DC) and the signal V_(SIN) occur at the same timeirrespective of the amplitude variation in the AC input voltage V_(IN).As such, by comparing the signal V_(DC) with the signal V_(SIN), theLPWM generator 430 generates an LPWM signal with a substantiallyconstant duty cycle even if the AC input voltage V_(IN) varies.

The waveform 509 in FIG. 5 represents an example of an LPWM signalgenerated by the LPWM generator 430. The waveform 509 in FIG. 5represents an example of a dimming control signal DIM output from theAND gate 409. When the signal V_(SIN) is less than the signal V_(DC),the LPWM signal is logic low (OFF period). During an OFF period, thedimming control signal DIM output is logic low such that the controlswitch 411 is switched off. There is no current flowing through the LEDstring 340 during the OFF period. When the signal V_(SIN) is greaterthan the signal V_(DC), the LPWM signal is logic high (ON period).During an ON period, a feedback signal FB indicative of a currentthrough the LED string 340 is compared to the predetermined referencesignal V_(REF). The dimming control signal DIM is determined by a pulsesignal generated by the frequency generator 440 and a comparison resultof the feedback signal FB and the predetermined reference signalV_(REF). By way of example, the pulse signal from the frequencygenerator 440 can have a frequency between approximately 300 KHz and 2.5MHz. Controlled by the dimming control signal DIM, the control switch411 can be switched on and off alternately to regulate the LED currentflowing through the LED string 340.

In summary, embodiments in accordance with the present invention utilizea rectified AC signal V_(REC) to generate a signal V_(SIN) (such as thewaveform 501 or 505 in FIG. 5) proportional to the signal V_(REC) and asignal V_(DC) (such as the waveform 503 or 507 in FIG. 5) indicating anaverage voltage level of the signal V_(REC). Thus, an electrolyticcapacitor is removed. By removing the electrolytic capacitor, thedriving circuit according to embodiments of the present invention offersan enhanced power factor and a longer life time. Furthermore, bycomparing the signal V_(SIN) with the signal V_(DC) and comparing thefeedback signal FB with the predetermined reference signal V_(REF), arelatively constant illumination of the LED string 340 can be achievedregardless of the amplitude and/or frequency variations in the AC inputvoltage V_(IN).

FIG. 6 illustrates a flowchart 600 of a method for driving a lightsource, e.g., an LED string, according to one embodiment of the presentinvention. Although specific steps are disclosed in FIG. 6, such stepsare exemplary. That is, the present invention is well suited toperforming various other steps or variations of the steps recited inFIG. 6. FIG. 6 is described in combination with FIG. 3 and FIG. 4.

In block 601, a rectified AC voltage signal is converted to a firstsignal proportional to the rectified AC voltage signal. The rectified ACvoltage signal is further converted to a second signal proportional toan average voltage level of the rectified AC voltage signal. In oneembodiment, the voltage converter 320 first converts an AC input voltagesignal V_(IN) to a rectified AC voltage V_(REC). The voltage converter320 further converts the V_(REC) to a first signal V_(SIN) and a secondsignal V_(DC). The signal V_(SIN) is proportional to the signal V_(REC)and the signal V_(DC) is proportional to an average voltage level of thesignal V_(REC).

In block 603, the rectified AC voltage signal is also converted to aregulated DC voltage signal to drive the light source. In oneembodiment, the voltage converter 320 converts the signal V_(REC) to aregulated DC output voltage V_(OUT) to drive the LED string 340.

In block 605, the first signal is compared with the second signal togenerate a pulse signal. In one embodiment, the LPWM generator 430compares the first signal V_(SIN) with the second signal V_(DC) togenerate an LPWM signal. In one embodiment, the LPWM signal has asubstantially constant duty cycle regardless of the amplitude and/orfrequency variations in the AC input voltage V_(IN).

In block 607, a dimming control signal is generated based on the pulsesignal to control dimming of the light source. In one embodiment, basedon the LPWM signal and a comparison result between a reference signaland a feedback signal indicating a current through the LED 340, the ANDgate 409 provides the dimming control signal DIM to the control switch411 to control the dimming of the LED string 340 accordingly.

While the foregoing description and drawings represent embodiments ofthe present invention, it will be understood that various additions,modifications and substitutions may be made therein without departingfrom the spirit and scope of the principles of the present invention asdefined in the accompanying claims. One skilled in the art willappreciate that the invention may be used with many modifications ofform, structure, arrangement, proportions, materials, elements, andcomponents and otherwise, used in the practice of the invention, whichare particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims and theirlegal equivalents, and not limited to the foregoing description.

What is claimed is:
 1. A circuit for driving a light emitting diode(LED) light source, said circuit comprising: a voltage converteroperable for converting an alternating current (AC) input voltage signalto a first rectified AC voltage signal, and for generating an averagesignal proportional to an average voltage level of said first rectifiedAC voltage signal; a switch coupled to said LED light source in series;and a controller coupled to said voltage converter and said switch andoperable for comparing said first rectified AC voltage signal with saidaverage signal to generate a pulse signal, and for generating a dimmingcontrol signal according to said pulse signal to control said switchthereby controlling dimming of said LED light source.
 2. The circuit ofclaim 1, wherein said pulse signal comprises a pulse width modulationsignal having a substantially constant duty cycle regardless of anamplitude variation in said AC input voltage signal.
 3. The circuit ofclaim 1, wherein said pulse signal comprises a pulse width modulationsignal having a substantially constant duty cycle regardless of afrequency variation in said AC input voltage signal.
 4. The circuit ofclaim 1, wherein said voltage converter comprises: a rectifier operablefor rectifying said AC input voltage signal to a second rectified ACvoltage signal; a first voltage divider coupled to said rectifier andoperable for dividing said second rectified AC voltage signal to providesaid first rectified AC voltage signal; and a second voltage dividercoupled to said rectifier and operable for dividing said secondrectified AC voltage signal to provide said average signal, wherein saidaverage signal is also proportional to an average voltage level of saidsecond rectified AC voltage signal.
 5. The circuit of claim 4, whereinsaid first voltage divider and said second voltage divider comprise aplurality of resistors, and wherein a duty cycle of said pulse signal isdetermined by resistance of said resistors.
 6. The circuit of claim 1,wherein said controller further comprises a comparator operable forreceiving a feedback signal indicative of a current flowing through saidLED light source and for comparing said feedback signal with apredetermined reference signal.
 7. The circuit of claim 6, wherein saidpredetermined reference signal indicates a peak current flowing throughsaid LED light source.
 8. The circuit of claim 6, further comprising: anAND gate coupled to said comparator and operable for generating saiddimming control signal based on said pulse signal and an output of saidcomparator.
 9. The circuit of claim 1, wherein said voltage converterconverts said AC input voltage signal to a regulated voltage to drivesaid LED light source.
 10. The circuit of claim 1, wherein said voltageconverter does not include an electrolytic capacitor.
 11. The circuit ofclaim 1, wherein said controller further comprises a frequency generatoroperable for generating a frequency signal, and wherein said dimmingcontrol signal is generated based on said frequency signal and saidpulse signal.
 12. The circuit of claim 11, wherein said switch isswitched on and off alternately according to said frequency signal and afeedback signal indicating a current flowing through said LED lightsource during an on period of said pulse signal, and wherein said switchremains off during an off period of said pulse signal.
 13. A systemcomprising: a voltage converter coupled to a light emitting diode (LED)light source and operable for converting an AC input voltage signal to aregulated voltage signal to drive said LED light source and forgenerating a first output voltage signal and a second output voltagesignal based on said AC input voltage signal; and a controller coupledto said voltage converter and operable for comparing said first outputvoltage signal to said second output voltage signal to generate a pulsesignal having a substantially constant duty cycle regardless of avariation in said AC input voltage signal, and for comparing a feedbacksignal indicative of a light source current with a predeterminedreference signal to generate a comparison signal, wherein saidcontroller is further configured to generate a control signal based onsaid pulse signal and said comparison signal to regulate power deliveredto said LED light source.
 14. A controller for regulating the brightnessof a light emitting diode (LED) light source, said controllercomprising: a first voltage input pin operable for receiving a firstvoltage signal proportional to a rectified AC signal of a power supplyof said LED light source; a second voltage input pin operable forreceiving a second voltage signal proportional to an average voltage ofsaid rectified AC signal; a sense pin operable for receiving a feedbacksignal indicative of a current flowing through said LED light source,wherein said controller is configured to generate a pulse widthmodulation (PWM) signal by comparing said first voltage signal with saidsecond voltage signal, and to provide a pulse signal according to afrequency signal and a comparison result of said feedback signal and areference signal, and to generate a dimming control signal based on saidPWM signal and said pulse signal to regulate said brightness.
 15. Thecontroller of claim 14, further comprising: a pin operable fordetermining a frequency of said frequency signal.
 16. The controller ofclaim 14, further comprising: a switch coupled to said LED light sourcein series and operable for receiving said dimming control signal andbeing switched on and off according to said dimming control signal. 17.The controller of claim 14, further comprising: a switch coupled to saidLED light source in series, wherein said switch is switched on and offalternately according to said frequency signal and said feedback signalduring an ON period of said PWM signal, and wherein said switch is offduring an OFF period of said PWM signal.
 18. The controller of claim 14,further comprising: a flip-flop operable for receiving said frequencysignal and for receiving said comparison result of said feedback signaland a reference signal, and for providing said pulse signal.
 19. Thecontroller of claim 14, further comprising: an AND gate operable forreceiving said PWM signal and said pulse signal, and for providing saiddimming control signal.
 20. The controller of claim 14, wherein saidreference signal indicates a peak current flowing through said LED lightsource.